FIG. 1 is a simplified schematic diagram of a conventional FPGA 110. FPGA 110 includes user logic circuits such as input/output blocks (IOBs) 160, configurable logic blocks (CLBs) 150, and programmable interconnect 130, which contains programmable switch matrices (PSMs). Each IOB 160 includes a bonding pad (not shown) to connect the various user logic circuits to pins (not shown) of FPGA 110. Some FPGAs separate the bonding pad from the IOB and may include multiple IOBs for each bonding pad. Each IOB 160 and CLB 150 can be configured through configuration port 120 to perform a variety of functions. Configuration port 120 is typically coupled to external pins of FPGA 110 through various bonding pads to provide an interface for external configuration devices to program the FPGA. Programmable interconnect 130 can be configured to provide electrical connections between the various CLBs and IOBs by configuring the PSMs and other programmable interconnect points (PIPS, not shown) through configuration port 120. IOBs can be configured to drive output signals to the corresponding pin of the FPGA, to receive input signals from the corresponding pins of FPGA 110, or to be bi-directional.
FPGA 110 also includes dedicated internal logic. Dedicated internal logic performs specific functions and can only be minimally configured by a user. Configuration port 120 is one example of dedicated internal logic. Other examples may include dedicated clock nets (not shown), delay lock loops (DLL) 180, block RAM (not shown), power distribution grids (not shown), and boundary scan logic 170 (i.e. IEEE Boundary Scan Standard 1149.1, not shown).
FPGA 110 is illustrated with 16 CLBs, 16 IOBs, and 9 PSMs for clarity only. Actual FPGAs may contain thousands of CLBs, thousands of PSMs, hundreds of IOBs, and hundreds of pads. Furthermore, FPGA 110 is not drawn to scale. For example, a typical pad in an IOB may occupy more area than a CLB, or PSM. The ratio of the number of CLBs, IOBs, PSMs, and pads can also vary.
FPGA 110 also includes dedicated configuration logic circuits to program the user logic circuits. Specifically, each CLB, IOB, and PSM contains a configuration memory (not shown) which must be configured before each CLB, IOB, or PSM can perform a specified function. Typically, the configuration memories within an FPGA use static random access memory (SRAM) cells. The configuration memories of FPGA 110 are connected by a configuration structure (not shown) to configuration port 120 through a configuration access port (CAP) 125. A configuration port (a set of pins used during the configuration process) provides an interface for external configuration devices to program the FPGA. The configuration memories are typically arranged in rows and columns. The columns are loaded from a frame register which is in turn sequentially loaded from one or more sequential bitstreams. (The frame register is part of the configuration structure referenced above.) In FPGA 110, configuration access port 125 is essentially a bus access point that provides access from configuration port 120 to the configuration structure of FPGA 110.
FIG. 2 illustrates a conventional method used to configure FPGA 110. Specifically, FPGA 110 is coupled to a configuration device 230, such as a serial programmable read only memory (SPROM), an electrically programmable read only memory (EPROM), or a microprocessor. Configuration port 120 receives configuration data, usually in the form of a configuration bitstream, from configuration device 230. Typically, configuration port 120 contains a set of mode pins, a clock pin and a configuration data input pin. Configuration data from configuration device 230 is typically transferred serially to FPGA 110 through a configuration data input pin. In some embodiments of FPGA 110, configuration port 120 comprises a set of configuration data input pins to increase the data transfer rate between configuration device 230 and FPGA 110 by transferring data in parallel. Further, some FPGAs allow configuration through a boundary scan chain. Specific examples for configuring various FPGAs can be found on pages 4–46 to 4–59 of “The Programmable Logic Data Book”, published in January, 1998 by Xilinx, Inc., and available from Xilinx, Inc., 2100 Logic Drive, San Jose, Calif. 95124, which pages are incorporated herein by reference.
Design engineers incorporate FPGAs into systems due to the flexibility provided by an FPGA. Because FPGAs are programmable and re-programmable, a design engineer can easily accommodate changes to the system specification, correct errors in the system, or make improvements to the system by reprogramming the FPGA. However, once the system design is complete, the flexibility provided by the programmability of an FPGA is sometimes not required. Furthermore, because FPGAs are relatively costly ICs and FPGAs require a configuration device which also increases cost, mass produced systems may not tolerate the cost of including FPGAs. Thus, in some systems that are mass produced, FPGAs used in the design phase of the system are replaced by less costly integrated circuits.
Most FPGA manufacturers provide a method to convert an FPGA design into a less costly integrated circuits. For example, some FPGA manufacturers replace the programmable elements of an FPGA with metal connections based on the design file of the FPGA to produce a mask programmed IC. All other circuitry remains the same between the mask programmed IC and the FPGA. The mask programmed IC is cheaper to manufacture than the FPGA and eliminates the need for the configuration device in the mass produced system. However, the mask programmed IC may still be more costly than desired because the semiconductor area, which is a major factor in the cost of an IC, required by the mask programmed IC is nearly the same as the FPGA. Consequently, the manufacturing cost of the mask programmed IC is not significantly cheaper than the FPGA.
Some manufacturers use a “sea-of-gates” approach to map an FPGA design into an application specific integrated circuit (ASIC). Specifically, the used CLBs, IOBs, memory cells, and programmable interconnect logic of the FPGA are mapped into corresponding areas of a gate array base. See for example U.S. Pat. No. 5,550,839 entitled “Mask-Programmed Integrated Circuits Having Timing and Logic Compatibility to User-Configured Logic Arrays” and U.S. Pat. No. 5,815,405 entitled “Method and Apparatus for Converting a Programmable Logic Device Representation of a circuit into a second representation of the circuit.” However, “sea-of-gates” gate arrays are not well suited to reproduce the extensive routing and other circuits available in an FPGA. Thus, gate array implementation of FPGA designs may prove costly for FPGA designs requiring extensive routing. Hence, there is a need for a method and structure to convert an FPGA design into an integrated circuit which minimizes the cost of the integrated circuit by reducing the size of the integrated circuit.